1. Field of the Invention
The present invention relates to a watermark data insertion method for inserting watermark data into a digitalized signal representing images, sounds, and/or the like to indicate their copyrights, and a watermark data detection method for detecting inserted watermark data.
2. Description of the Related Art
Techniques for inserting watermark data into a digitalized signal representing images, sounds, and/or the like to indicate their copyrights having been vigorously studied and developed. Such watermark data is required to have minimum interference with the images and/or sounds of interest, be hardly visible (in the case of image data) and hardly audible (in the case of sound data), and yet be easily detected by a person knowing the detection method while preventing unauthorized removal of the watermark data by third parties. By using such watermark data, for example, infringement of a copyright (by illegal copying of images and/or sounds) by unauthorized third parties can be established or prevented.
One conventional method for inserting watermark data into an original signal representing images, sounds, and/or the like utilizes a spread spectrum technology. In principle, this method involves: multiplying the original signal representing images, sounds, and/or the like by random numbers to obtain a uniform spectrum; adding watermark data to the multiplied signal; and again multiplying the resultant signal with the same random numbers to restore the original signal. At the time of restoring the original signal, the added watermark data is also multiplied by the random numbers. As a result, the watermark data has a uniformly spread spectrum, i.e. becomes white noise, in the restored original signal.
FIG. 7 is a block diagram illustrating the structure of a conventional watermark data insertion apparatus utilizing a spread spectrum technology. As shown in FIG. 7, an original signal al (into which watermark data x.sub.i is to be inserted) is input to a multiplier 52, where it is multiplied by predetermined pseudo random numbers to become a.sub.i.times.c.sub.i. Since the pseudo random numbers c.sub.i are a random sequence of +1 and -1, the frequency spectrum of a.sub.i.times.c.sub.i becomes uniform.
Next, the watermark data x.sub.i is input to an adder 53 to be added to a DC component of the above multiplication product a.sub.i.times.c.sub.i. Although not shown, the adder 53 includes a cascade connection of a Fourier transformer, an adder, and an inverse Fourier transformer for extracting a DC component of an input signal.
The signal a.sub.i.times.c.sub.i +x.sub.i (i.e., the randomized original signal plus the watermark data) is input to a multiplier 54, where it is again multiplied by the same pseudo random numbers to give a.sub.i.times.c.sub.i.sup.2 +x.sub.i.times.c.sub.i. Since the pseudo random numbers are +1 or -1, c.sub.i.sup.2 equals 1. Therefore, a signal having a value of a.sub.i +x.sub.i.times.c.sub.i (=a.sub.i.times.c.sub.i.sup.2 +X.sub.i.times.c.sub.i) is obtained, i.e., a signal with embedded watermark data.
FIG. 8 is a block diagram illustrating the structure of a conventional watermark data detection apparatus utilizing a spread spectrum technology for detecting embedded watermark data. As shown in FIG. 8, a signal with embedded watermark data a.sub.i +x.sub.i.times.c.sub.i is input to a multiplier 62, where it is multiplied by predetermined pseudo random numbers to become a.sub.i.times.c.sub.i +x.sub.i.times.c.sub.i.sup.2 =a.sub.i.times.c.sub.i +x.sub.i. Since the pseudo random numbers ci are a uniform random sequence of +1 and -1, the DC component of a.sub.i.times.c.sub.i becomes zero, so that only the watermark data x.sub.i is extracted by an extractor 63. The extractor 63 includes a Fourier transformer for extracting a DC component of an input signal.
The above-described technique is reported in "Watermark data signature method for images utilizing PN series", Electronic Information Communication Society, "Encryption and Information Security Symposium" proceedings, SCIS '97-26B, 1997.
However, the above-described conventional technique has a disadvantage in that the watermark data may become noticeable where the original signal has a small amplitude because the watermark data is inserted into the original signal in a uniform manner, i.e., regardless of the characteristics of the original signal. There is also a disadvantage in that, if the amplitude of the watermark data is decreased in order to prevent the watermark data from becoming noticeable, the watermark data may be masked by the original signal and therefore difficult to detect. Furthermore, unauthorized third parties can make the watermark data impossible to detect by intentionally adding noise or the like to the original signal.